Amplifier circuits



y 1934- w. F. MARRIAGE 51' AL 1,960,019

MPLIFIER CIRCUITS Filed Dec. 23. 1929 l WWW SUPPBESSOB 007F117 VOLTQGE W EMARR/ABZ' WVE/YTDHS F? f?- THOMAS BY 0 .s'z/Ppzzssaz nvpur VOLT46 to disable the east repeater.

Patented May 22, 1934:

NETED STATES PATENT QFFICE AMPLIFIER CIRCUITS Application December 23, 1929, Serial No. 415,888 In Great Britain February 14, 1929 5 Claims. (01. 179-170 The present invention is concerned with improvements in amplifier circuits and particularly with improvements in amplifier circuits employing echo suppressors of the grid-jamming type.

An object of the invention is to improve the operation of signal-controlled amplifier circuits.

Echo suppressors of the grid-jamming type usually comprise an amplifier-rectifier by means of which speech currents are rectified to produce a D. C. voltage output. This D. C. voltage is applied to the grid of a repeater valve, which is normally biased to its operating voltage, in such a manner as to render the valve inoperative, or in the case where the repeater valve is normally biased to a point where it is inoperative, the bias of the repeating valve is brought to such a value that the repeating valve is rendered operative.

The operation of these two systems may be described as follows, the terms east and west being used to discriminate between speech in the two directions. 7

In the first case the repeaters are biased so as to be in the operative condition in the absence of signals. The input to the echo suppressor is connected to some point in the. speech transmission circuit. Thus when speech is being transmitted in the east direction, the corresponding echo suppressor operates to give a D. C. voltage which is applied in a negative sense to the grid of the west repeater valve. This repeater is then disabled and does not transmit echo currents. When speech is being transmitted in the west direction, the corresponding echo suppressor operates in a similar manner In order to allow for the time of transmission of speech echos back from the furthest point in the circuit at which echoes may originate to the point at which the echo suppressors are situated, it is necessary that the repeater shall remain inoperative for a certain time interval after the cessation of speech. This is usually obtained by the use or" a condenser and resistance arrangement which has a definite discharge time between two voltages.

In a known arrangement the voltage applied to the condenser resistance arrangement varies according to the energy level of the signals, thus rendering the inoperative time, after cessation of the signals (i.e. the hangover time), a iunction of the energy level of the signals. This is undesirable.

In the second case the repeaters are normally biased so that they are inoperative. The echo suppressor is so connected that when speech is transmitted in the east direction a positive bias is applied in opposition to the bias battery of the east repeater valve. This brings the resulting bias on the repeater valve to the normal operating value and so enables speech to be transmitted. A delay circuit may be used between the point at which the echo suppressor is connected to the line, and the input to the repeater in order to compensate for the time required for the echo suppressor to bring the repeater into full operation. In this case it is necessary that the D. C. voltage supplied by the echo suppressor shall be constant in order that the repeater valve may operate on the correct portion of its characteristic. In order as far as possible to avoid false operation of the echo suppressor by line noiseor crosstalk it is desirable that the increase in input level required to raise the D. C. output voltage from zero to the constant maximum value shall be as small as possible.

According to the invention, means are provided for rendering the hang-over time (i.e. lapse of time during which the repeater associated with one direction of transmission remains inoperative when transmission has ceased in the opposite direction) of an echo suppressor of the grid-jamming type substantially independent of the energy level of the signals, and according to one feature of this invention, means is provided for charging the condenser of a resistance-condenser hangover, arrangement under a constant voltage whereby the duration of the period of incperativeness after cessation of the signals is rendered constant. A further feature of the invention consists in an amplifier rectifier as described above so designed that the increase in input energy level required to raise its D. C. output voltage from Zero to the maximum value shall be small.

An arrangement according to this invention may also be applied to give a constant D. 0. Voltage to bring into operation repeaters of the type which are normally inoperative.

The invention will be better understood from the following description and accompanying drawing in which Fig. 1 is a circuit diagram showing a 2-Way, 4-wire repeater with associated echo suppressors which embody the invention, while Fig. 2 shows characteristics 105 curves of the echo suppressor.

Referring more particularly to Fig. 1 there is shown part of a l-wire circuit comprising lines 1, l and 2, 2' in which are connected repeaters 3 and 4, the repeater 3 being for transmission in one direction while the repeater 4 transmits in the other. Associated with the repeaters 3, 4 are echo suppressors 5, 6.

Consider now echo suppressor 5 which operates to disable repeater 3 when speech is being transmitted on line 2, 2. There is a tuned transformer input '7 to the three electrode vacuum tube 8 which is biased to operate as an amplifier in the most efiicient manner. The anode circuit of the vacuum tube 8 is coupled by means of a transformer 9 to vacuum tube 10 which is biased to operate as an anode bend rectifier. The anode circuit of the vacuum tube 10 is supplied with battery through resistance 11 and the primary winding of transformer 12. The secondary winding of transformer 12 is connected to the anode-grid circuit of vacuum tube 13 which operates as a diode rectifier into resistance 14 in the parallel resistance and condenser combination forming the hangover circuit.

The amplifying and rectifying valves are supplied With the usual batteries for filament heating and plate voltage.

By the circuit arrangement which has just been described, it is possible to make the rectified current in the output of rectifier 13, and therefore the D. C. voltage available across resistance 14 increase suddenly and finally reach a steady state, when speech currents are supplied to the input of the suppressor. It is, however, desirable that the echo suppressor does not operate when the level of the speech current does not attain a predetermined value. The circuit elements can be so chosen and adjusted in the above described suppressor that the interference currents are below the speech level required to produce full operation of the suppressor.

Some of the aforementioned desirable features are obtained in this suppressor by the use of resistance 11, the action of which will now be described. A choke coil 17 is connected to the plate of the valve 8 and to one terminal of resistance 11.

Speech currents entering the suppressor are amplified by the amplifier valve 8 and are applied to the grid of the valve 10.

Owing to the large negative grid bias on valve 10 the negative halves of the applied voltage swing the grid so far negative that a negligible change in anode current is produced. The positive halves of the applied voltage, however, produce considerable changes in the anode current, and the steady value of this current is thus increased in accordance with the amplitude of the applied voltage. The alternating component of the anode current of tube 10 passes through the primary winding of trans former 12 and condenser 15 to the filament of valve 10. This produces a voltage in the secondary of transformer 12 which is applied to the anode rectifier 13 and resistance 14.

The direct current component of the anode current of valve 10 passes through the primary of transformer 12 and resistance 11. This current through resistance 11 causes a fall in the voltage applied to the anode of valve 10; this fall results in a reduction of the amplification of the valve 10 which reduction is increased as the input to valve 10 increases. By suitably choosing the value of resistance 11 the fall in amplification can be made to compensate for the increase of input, thus maintaining the voltage applied to the valve 13 and resistance 14 substantially steady. The resulting steady current in the output of the rectifying tube 13 is transmitted to the resistance 14 and the condenser in parallel therewith. The resulting voltage drop in resistance 14 located in the gridcathode circuit of the valve in the second stage of the repeater 3 applies a biasing potential to the grid of that valve' of such value as to render it inoperative.

On the cessation of the supply of speech currents from the repeating paths 2, 2' to the input of the suppressor 5, the amount of output current from the rectifier tube 13 passing through the resistance 14 is reduced to zero. However, the repeater valve 3 does not immediately return to its normal operative condition because the condenser in shunt with the resistance 14 Which has been charged up from the output of the rectifier 13 during the time when speech currents are impressed on the input of the suppressor 5, on cessation of these speech currents, will discharge gradually through the resistance 14. Thus, the biasing potential on the grid of the repeater valve 3 rendering that valve inoperative is maintained for an additional hangover time interval. The interval of time which elapses before the biasing potential on the grid of the repeater valve 3 returns to its normal value, depends on the value of the capacity of the condenser, the value of the resistance 14 and the initial voltage at which they are charged. This hangover interval is not a function of the energy level of the signals supplied to the input of the suppressor because of the action of the resistance 11 in preventing variations in the input level from being applied to the input of the rectifier tube 13.

By inserting the battery 16 of suitable value in series with resistance 14 in the circuit of valve 13 in such a Way as to oppose the voltage generated by valve 13 and transformer 12 it is possible to insure that for inputs to the suppressor below a given level, there shall be no voltage produced across resistance 14; this gives protection against possible partial operation by interference currents of a level lower than the level of the speech currents desired to operate the suppressor.

In a similar manner echo suppressor 6 operates to disable repeater 4 when speech is being transmitted on line 1, 1.

Echo suppressors of the type described may also be applied to repeatered 2-wire telephone circuits for instance by taking advantage of the fact that at points where repeaters are installed each 2-wire circuit becomes a local 4-wire circuit.

In Fig. 2 are shown characteristic curves indicating the variation in the voltage obtained across resistance 14 as the input voltage to the suppressor varies for different values of resistnce 11. The abscissae represents the input voltages and the ordinates the output voltages. Curve 1 is for the case where resistance 11 is zero, curve 3 is for the case of a suitable value of resistance 11 while curve 2 shows the characteristic obtained for an intermediate value of resistance 11.

What is claimed is:

1. In combination in a two-Way signal transmission system two one-way transmission paths each including an amplifier, for the signals transmitted in opposite directions, means for applying to the amplifier in one one-way path a biasing voltage derived from the signals transmitted over the other one-way path. in such manner as to disable that amplifier, and to maintain it disabled for a definite hangover interval after signal transmission in said other one-way pathhas ceased, and means for making the length of the hangover interval substantially independent of variations in the energy level of the signals in said other path, comprising means for suppressing such variations from the derived voltage.

2. The system oi claim 1 and in which the means for applying and maintaining the biasing voltage to disable the amplifier comprises a condenser and resistance in parallel in the input circuit of the amplifier, across which the derived voltage is applied, the duration of said hangover interval being determined by the discharge of said condenser through said resistance when signal transmission in said other path ceases.

3. The system of claim 1 and in which the means for applying and maintaining the biasing voltage to disable the amplifier comprises a condenser and resistance in parallel in the input circuit of the amplifier, and a vacuum tube amplifier-rectifier circuit having its input connected across said other path so as to receive waves therefrom, and its output connected across said condenser and resistance, and the means for making the duration of said hangover interval independent of variations in the energy level of the signals in said other path comprises means for making the amount of amplification produced by said vacuum tube circuit in the waves applied thereto automatically decrease as the energy level of said signals increases.

4. In combination in a two-way signal transmission system, two one-Way transmission paths for the signals transmitted in opposite directions over the system, an amplifier in each one-way path, a resistance and condenser in parallel connected in the input circuit of each amplifier, a vacuum tube circuit connected to one oneway path in the output of the amplifier therein and responsive to signals transmitted over said one one-way path for applying a rectified biasing potential across said resistance and condenser in the input circuit of the amplifier in the other one-way path such as to change the operation characteristics of that amplifier, and means automatically responsive to the signal waves impressed on said vacuum tube circuit from said one one-way path to control the amplification factor of said vacuum tube circuit in such manner as to prevent variations in the energy level of the impressed signals from varying the amount of said biasing potential.

5. In combination in a two-way signal transmission system, one-way transmission paths each including an amplifier for the signals transmitted in opposite directions over the system, a condenser and a resistance in parallel in the input circuit of the amplifier in one oneway path, and means responsive to the signals transmitted over the other one-way path for applying to the input of said amplifier in said one one-way path across said resistance and condenser a biasing potential which is substantially constant with variations in the amplitude of the controlling signals, said means comprising two three-electrode vacuum tubes each having a plate electrode, connected in tandem, the input circuit of the first tube being connected to said other one-way path and the output circuit of the second tube being coupled through a rectifying device across said condenser and resistance, the plate electrodes of said two vacuum tubes being connected through a resistance and a choke coil.

WILLIAM FRANK MARRIAGE. PHILIP ROBERT THOMAS. 

